If you have heard that it is always recommended choose Dedicated Servers for Exchange hosting then let me add that its not just a dedicated server that is recommended but a high end dedicated server with more than 4 Mb RAM is suggested.
This is because Exchange servers require Active directory and it backed up by SQL server as well. And Active Directory is not supported on VPS's and shared servers. Exchange server and SQL Server both require lot of resources,hence it is not possible to offer Exchange Hosting services on a VPS or Shared servers.Therefore,only the high end dedicated servers are recommended for hosting Microsoft's Exchange services.
I hope it helps.
